Typically in an engine mounted on a vehicle such as an automobile, a piston is connected to a small end portion as one end portion of a connecting rod via a piston pin, and a large end portion as the other end portion of the connecting rod is connected to a crank shaft. The small and large end portions of the connecting rod are connected together via a connecting portion of the connecting rod. Reciprocation of the piston is transmitted to the crank shaft via the connecting rod, thereby rotating the crank shaft.
It has been known that in the above-described engine, combustion noise is caused due to resonance determined by the basic structure of the engine (see, e.g., Non-Patent Document 1). Fast combustion made at a diesel engine or an engine capable of performing homogeneous-charge compression-ignition combustion (HCCI) amplifies vibration with a frequency of 1 kHz to 2 kHz and vibration with a frequency of 3 kHz to 4 kHz, resulting in knocking sound. In Non-Patent Document 1, engine sound has three peaks of 1.7 kHz, 3.3 kHz, and 6 kHz.
One (3.3 kHz) of these peaks is caused due to extension/contraction resonance of the connecting rod. That is, in the spring mass model including the piston and the connecting rod, the piston, the piston pin, and the small end portion of the connecting rod correspond, as a whole, to a mass point, and the connecting portion of the connecting rod corresponds to a spring supporting the mass point. With this configuration, when the piston, the piston pin, and the small end portion of the connecting rod are regarded as an integrated portion, such an integrated portion resonates on the large end portion of the connecting rod. Such resonance corresponds to the extension/contraction resonance of the connecting rod as described in Non-Patent Document 1. The following technique for reducing the extension/contraction resonance has been filed as an application (Japanese Patent Application No. 2012-189134) by the applicant of the present invention: a dynamic absorber is provided in a piston pin to suppress a piston, the piston pin, and a small end portion of a connecting rod from resonating together.
On the other hand, the inventor(s) of the present invention has made the following assumption regarding the vibration with a frequency of 1 kHz to 2 kHz. The piston, the piston pin, and the connecting rod correspond, as a whole, to a mass point in the spring mass model including the piston, the connecting rod, and the crank shaft. Extension/contraction is made between the crank shaft and the large end portion of the connecting rod, and therefore, the portion between the crank shaft and the connecting rod corresponds to a spring. However, there has been no countermeasure against such resonance. Due to improvement of the extension/contraction resonance (3.3 kHz) of the connecting rod, the resonance with a frequency of 1 kHz to 2 kHz becomes more noticeable. Another countermeasure against such resonance is required.
Patent Document 1 discloses the following structure. In an annular washer which is fitted onto a leg portion of a bolt screwed into a connecting rod and which is fastened and fixed to the connecting rod via the bolt, a housing space closed from an external space is formed inside the washer. In the housing space, a separate mass member is housed and disposed so as to independently displace in a non-bonded state. Thus, the separate mass member directly and elastically contacts an inner surface of the housing space.